1. Field of the Invention
The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display and an overdrive method thereof.
2. Discussion of the Related Art
In general, a liquid crystal display controls the light transmittance of liquid crystal cells using an electric field to display a picture. Liquid crystal displays include a thin film transistor substrate and a color filter substrate that are opposed to each other with a liquid crystal disposed therebetween.
To display a picture, a liquid crystal display typically controls the light transmittance of liquid crystal cells in accordance with video signals. An active matrix type liquid crystal display includes a switching device for each liquid crystal cell. The switching device typically employed in the active matrix liquid crystal display is a thin film transistor (TFT). The active matrix type liquid crystal display is advantageous for the display of a moving picture.
One disadvantage of the liquid crystal display is a slow response time resulting from inherent characteristics of the liquid crystal, such as viscosity, elasticity, etc. Such characteristics can be explained by the following EQUATIONS 1 and 2.
                              τ          r                ∝                              γ            ⁢                                                  ⁢                          d              2                                            Δɛ            ⁢                                                                          V                  a                  2                                -                                  V                  F                  2                                                                                                      [                  EQUATION          ⁢                                          ⁢          1                ]            Here, ‘τr’ represents a rising time when a voltage is applied to a liquid crystal, ‘Va’ represents an applied voltage, ‘VF’ represents a freederick transition voltage at which liquid crystal molecules begin to perform a tilt motion, ‘d’ represents a cell gap of the liquid crystal cell, and ‘γ’ represents a rotational viscosity of the liquid crystal molecules.
                              τ          f                ∝                              γ            ⁢                                                  ⁢                          d              2                                K                                    [                  EQUATION          ⁢                                          ⁢          2                ]            Here, ‘τf’ represents a falling time at which a liquid crystal is returned to an initial position by an elastic restoring force after a voltage applied to the liquid crystal is turned-off, and ‘K’ represents an elastic constant.
A twisted nematic (TN) mode liquid crystal has a different response time due to physical characteristics of the liquid crystal, cell gap, etc. Generally, the TN mode liquid crystal has a rising time between 20 ms to 80 ms and a falling time between 20 ms to 30 ms. As the TN mode liquid crystal has a response time that is longer than one frame interval of a moving picture (i.e., 16.67 ms in the case of NTSC system), a moving picture is displayed with a brightness lower than the corresponding value of video data VD as shown in FIG. 1. As a result, a motion blurring phenomenon occurs.
As illustrated in FIG. 1, the related art liquid crystal display cannot present the desired color and brightness. Because of the slow response time of the related art liquid crystal displays, display brightness BL fails to achieve a target brightness corresponding to changes in the video data VD from one level to another level. Accordingly, the motion-blurring phenomenon occurs, and display quality deteriorates due to a reduction in a contrast ratio.
In order to overcome the problems posed by the slow response time of the liquid crystal display, U.S. Pat. No. 5,495,265 and PCT International Publication No. WO 99/05567 have suggested modulating data by using a look-up table (hereinafter, referred to as “overdriving method”). This overdriving method allows data to be modulated as shown in FIG. 2.
Referring to FIG. 2, the overdriving method of the related art liquid crystal display modulates input data VD and applies the modulated data MVD to the liquid crystal cell to obtain a desired brightness MBL. The related art overdriving method modulates |Va2−VF2| from the above EQUATION 1 on the basis of a difference so that a desired brightness can be obtained. The desired brightness corresponds to a brightness value of the input data within one frame interval. In this manner, the response time of the liquid crystal is reduced. Accordingly, a liquid crystal display employing the related art overdriving method compensates for the slow response time of the liquid crystal by modulating the input data in order to alleviate the motion blurring phenomenon. As a result, an image having a desired color and a desired brightness can be displayed.
The overdriving method of the related art liquid crystal display compares uppermost bit data MSB of the previous frame Fn−1 with uppermost bit data MSB of the present frame Fn. If the uppermost bit data MSB are changed, the overdriving method selects a modulation data Mdata provided in a look-up table as shown in FIG. 3. Furthermore, the overdriving method of the liquid crystal display only modulates several upper bits so as to reduce a capacitance of a memory upon realizing the overdriving method as a hardware device.
A configuration of an overdrive device implementing the related art overdriving method is depicted in FIG. 4. Referring to FIG. 4, an overdrive device 100 includes a frame memory 130 and a look-up table 140. The frame memory 130 is connected to an upper bit bus line 120, and the look-up table 140 is connected to output terminals of the upper bit bus line 120 and the frame memory 130. The frame memory 130 stores the uppermost bit data MSB for one frame period and supplies the stored data to the look-up table 140. The uppermost bit data MSB includes the upper four bits of the eight bit source data RGB.
The look-up table 140 compares upper bit data MSB of the present frame Fn with upper bit data MSB of the previous frame Fn−1 according to TABLE 1 to select a corresponding modulation data Mdata. The upper bit data MSB of the present frame Fn is inputted from the upper bit bus line 120, and the upper bit data MSB of the previous frame Fn−1 is inputted from the frame memory 130. The modulation data Mdata is added to bit data LSB from a lower bit bus line 110 to be supplied to the related art liquid crystal display.
TABLE 1 is a look-up table in which the four bits corresponding to the upper bit data MSB are represented by base 10 numbers. The left column represents data voltage VDn−1 of the previous frame Fn−1, and an uppermost row represents a data voltage VDn of the present frame Fn.
TABLE 1Division01234567891011121314150013467910 11 12 1415151515151012457910 11  13141515151520123578910 12 13141515151530123568910 11 12141415151540012467910 11 121314151515500023578911 121314151515600013468910 111314151515700012457810 1112141415158000123568911121314151590001234679101213141515100000124578101113141515110000023567 91112141515120000013457 8101213151513000001234  81011131415140000001235 7 911131415150000000124 6 911131415
The overdriving method of the related art liquid crystal display is more effective for reducing the liquid crystal response time when a gray scale is changed to a gray scale than when black is change to white. A change of the gray scale to gray scale has a lower voltage difference than a change from black to white. As a result, when the color is changed, the picture quality deteriorates.
Furthermore, the overdriving method of the related art liquid crystal display compares data of the previous frame Fn−1 with a data of the present frame Fn to generate modulation data MRGB. Since the overdrive device of the related art liquid crystal display includes a memory, such as a look-up table, manufacturing cost and chip size are increased.